Optical systems capable of forming images from illuminated objects find numerous and important applications in areas such as advertising, marketing and product exhibition, as well as other more esoteric functions such as simulation. It is advantageous for such systems to be compact in size, have a wide field of view and high contrast and be viewable in all ambient lighting conditions.
Many real image optical systems, e.g., real image projectors, are designed to create an image wherein the desired image appears against a black background. In a real image projector for, say, gaming applications, a floating, real image of a character is projected into space. The psychological impact of this image is greatest if the game elements appear bright, sharply defined and of high contrast. An example of a current state-of-the-art real image projector of this type is the dual brightness enhancement film (DBEF)-based meniscus-type projecting optical system disclosed in U.S. Pat. No. 6,262,841 to the present inventor and shown in FIG. 1 as optical system 20. However, optical system 20, while of very compact design, has a relatively low efficiency in terms of light throughput and exhibits poor contrast at non-normal, i.e., off-center or oblique, viewing angles.
Generally, the inclusion of DBEF-based optical system 20 herein serves to illustrate the basic function of a real image optical system, as well as to illuminate the drawbacks of that particular design relative to a real image optical system of the present invention. FIG. 1 shows optical system 20 as including a source light 24 emitted and/or reflected by a source 28. Source light 24 is randomly polarized in nature and is subsequently linearly polarized by a first linear polarizer 32 as it travels toward a viewer 36. This linearly polarized light passes through DBEF 40 having its axis aligned in the transmissive orientation with respect to the linearly polarized light passing through linear polarizer 32. The linearly polarized light that passes through DBEF 40 is circularly polarized by a first quarter wave retarder 44, or first quarter wave plate (QWP). This circularly polarized light (assumed right handed for this description) is then made incident upon a partially mirrored concave (spherical or aspherical) beamsplitter 48. Concave beamsplitter 48 serves to impart the convergence that ultimately forms the real image 52.
The reflected portion of the light has its handedness of circular polarization switched to left handedness by reflection at beamsplitter 48 and is converted to linearly polarized light by first quarter wave retarder 44, as it travels right to left in the figure. This linearly polarized light is largely reflected by DBEF 40, since the direction of linear polarization is opposite the polarization of the initial linearly polarized light. This portion of reflected light is then circularly polarized by first quarter wave retarder 44 as it now travels left to right in the figure. This light is partially reflected and partially transmitted by partially-mirrored concave beamsplitter 48. The transmitted portion is linearly polarized by a second quarter wave retarder 56. A second linear polarizer 60 is aligned such that the linearly polarized light is transmitted to form real image 52 apparent to viewer 36.
The portion of the right circularly polarized light transmitted through partially-mirrored concave beamsplitter 48 is converted to linearly polarized light by second quarter wave retarder 56 in a direction opposite to the polarization direction of the finally transmitted light. As the direction of the light that has passed through second quarter wave retarder 56 is opposite the transmissive direction of second linear polarizer 60, it is extinguished by the second linear polarizer.
Optical system 20 of FIG. 1 utilizes a series of polarizing components, i.e., DBEF 40 and first and second linear polarizers 32, 60 in conjunction with a beamsplitter, i.e., beamsplitter 48, in a compact design that redirects light several times to fold the optical path. This folding of the optical path results in a small device size and superior field of view. However, DBEF 40 and first and second linear polarizers 32, 60 are absorptive in nature and result in losses in terms of light throughput. Additionally, the contrast of the final image is affected, especially at oblique viewing angles, by the differing bandwidth responses among the polarizing elements. The contrast is primarily affected by the existence of undesired bleed-through of a portion of source light 24 resulting from poor extinction ratios of the polarizing elements in off-angle viewing conditions.
In addition to the shortcomings of optical system 20 just mentioned, there are other performance and manufacturing aspects which can be improved. For example, the type of quarter wave retarder used for first and second quarter wave retarders 44, 56 is of a drawn polyvinyl alcohol (PVA) type that characteristically exhibits poor retardance uniformity and has poor performance over time owing to inherent propensity of these types of polarizers to absorb water and thereby alter the retardance value. This lack of uniformity results in poor efficiency of the overall system that primarily manifests itself, again, as greater bleed-through at oblique viewing angles.
Optical system 20 also utilizes a flexible first quarter wave retarder 44 directly adjacent to DBEF 40, which itself is flexible. Since DBEF 40 is used in reflection along the desired light path, it must necessarily be flat to provide a distortion-free reflected image. However, laminating flexible DBEF 40 directly to flexible first quarter wave retarder 56 results in an undulating DBEF surface and, hence, a distorted reflection. The manufacturing complexity of maintaining flatness in flexible DBEF 40 while laminated (or optically coupled) to flexible first quarter wave retarder 44 on one side and first linear polarizer 32 on the other is apparent. Thus, DBEF 40 must be laminated to its own flat glass substrate (not shown) prior to lamination to first quarter wave retarder 44 and additional polarizing and anti-reflective glass elements. This additional step results in optical system 20 including a subassembly having three rigid substrates, including two anti-reflective glass components, along with three sheet-type polarizing elements, resulting in a substantial manufacturing complexity. Regarding DBEF 40 itself, the stand-alone contribution of this element to the overall throughput of the entire system is about 49%, i.e., about 70% reflection and about 70% transmission.
The portion of the right circularly polarized light that is ideally extinguished by second linear polarizer 60 is usually not sufficiently, i.e., “cleanly,” polarized to be completely extinguished by this polarizer. Inefficiencies exist, since current PVA-type polarizers are optimized at only a single wavelength, whereas source light 24 for the intended applications, e.g., the applications discussed above, is typically polychromatic. Oblique viewing of real image 52 further tests the limitations of the current quarter wave retarders, since the performance of this type of retarder is highly viewing-angle dependent.
Accordingly, it is desired to obtain a compact meniscus-type real image projector having higher brightness and contrast and better manufacturability than optical system 20, while retaining or improving the superior system size and field of view characteristics of that system. Several improvements for enhancing the image characteristics of optical system of are disclosed herein.